Probe for combined signals

ABSTRACT

A direct current and a modulation signal are simultaneously applied to contact pads on a device under test, such as a laser diode, with a probe that reduces signal distortion and power dissipation by transmitting a modulated signal through an impedance matching resistor and transmitting of a direct current over a second signal path that avoids the impedance matching resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 11/077,810, U.S. Pat. No.(7,075,320) filed Mar. 9, 2005; which is a continuation of applicationSer. No. 10/928,688, U.S. Pat. No. (7,046,023) filed Aug. 26, 2004;which is a continuation of application Ser. No. 10/712,579, U.S. Pat.No. (6,806,724) filed Nov. 12, 2003, now U.S. Pat. No. 6,806,724 B2;which is a division of application Ser. No. 10/294,130 U.S. Pat. No.(6,724,205), filed Nov. 13, 2002, now U.S. Pat. No. 6,724,205 B1.

BACKGROUND OF THE INVENTION

The present invention relates to probe measurement systems for measuringthe electrical characteristics of integrated circuits and othermicroelectronic devices tested by simultaneously applying a directcurrent and a modulation signal to the device-under-test.

There are many types of probing assemblies that have been developed formeasuring the characteristics of integrated circuits and other forms ofmicroelectronic devices. One representative type of assembly uses acircuit card on which are formed elongate conductive traces that serveas signal and ground lines. A central opening is formed in the card, anda needle-like probe tip is attached to the end of each trace adjacentthe opening so that a radially extending array of downwardly convergingneedle-like tips is presented by the assembly for selective connectionwith the closely spaced contact pads of the microelectronic device beingtested. A probe assembly of this type is shown, for example, in HarmonU.S. Pat. No. 3,445,770. This type of probing assembly, however, isunsuitable for use at higher frequencies, including microwavefrequencies in the gigahertz range, because at such frequencies theneedle-like tips act as inductive elements and because there are noadjoining elements present to suitably counteract this inductance with acapacitive effect in a manner that would create a broadbandcharacteristic of more or less resistive effect. Accordingly, a probingassembly of the type just described is unsuitable for use at microwavefrequencies due to the high levels of signal reflection and substantialinductive losses that occur at the needle-like probe tips.

One type of probing assembly that is capable of providing acontrolled-impedance low-loss path between its input terminal and theprobe tips is illustrated in Godshalk et al., U.S. Pat. No. 5,506,515.The probe has a tip assembly including a semi-rigid coaxial cable with aTeflon™ dielectric and a freely-suspended end. An inner finger and anouter pair of fingers are mounted on the freely-suspended end of thecable. Each of the fingers is made of resilient conductive material, soas to form a coplanar transmission line. Cantilevered portions of thefingers extend past the end of the cable to form an air-dielectrictransmission path of uniform and stable characteristics despite exposureto numerous contact cycles. The fingers provide a suitable means forprobing nonplanar wafer contact pads while promoting good visibility inthe area of the contact pads. The characteristic impedance of typicalmicrowave probes and cables is approximately 50 ohms closely matchingthe impedance of the typical microwave device-under-test (DUT) sobroadband signals can travel through the probe with minimal loss.

However, when testing certain devices, such as laser diodes, the use ofa typical microwave probe is problematic. Laser diode testing requiressimultaneous application of a modulation signal and a DC electricalcurrent to a contact pad of the device to generate a modulated lightoutput. For testing, the modulation signal is typically a sweptfrequency sinusoid (AC) or a wide bandwidth pulsed waveform. The DC andmodulation signals are superimposed and the combined signals areconducted to a contact tip of a probe in selective engagement with thecontact pad of the DUT. Typically, the impedance seen by the modulationsignal, the dynamic resistance of an active laser diode, for example, ison the order of five ohms. As a result, there is a significant impedancemismatch with the typical microwave probe and cable and the mismatchedimpedance distorts the modulation signal measured by the testinstrumentation. While some instrumentation, such as a Vector NetworkAnalyzer (VNA), can be calibrated to correct for distortion, thedistortion can substantially affect measurements made with otherinstrumentation. Further, the distortion can have a magnitude sufficientto attenuate the modulation signal at some frequencies, causing a lossof dynamic range and accuracy for the measurements, even when made witha VNA.

To improve the impedance matching and reduce distortion of themodulation signal, an impedance matching resistor can be installed inseries with the contact tip of a microwave probe. For testing laserdiodes, the typical series impedance matching resistor has a value of 45ohms, which in series with the five ohm dynamic resistance of a typicallaser diode, provides a satisfactory impedance match with the probes andcables (≈50 ohms) to substantially reduce distortion of the testsignals. Resistors with other values can be incorporated into the probeto match impedance when testing other types of devices. However, sincethe modulation signal and the DC current are superimposed on the sameconductor, both signals must pass through the series impedance-matchingresistor which dissipates power equal to the product of the resistanceand the square of the current. For DUTs requiring higher current levels,the power that must be dissipated by the resistor is substantial. On theother hand, to pass high frequency signals, the resistor must small insize and the distance between the resistor and the contact tip must beshort to minimize parasitic series inductance and shunt capacitance. Theperformance of a probe with a series impedance matching resistor iscompromised by the competing necessities of sufficient resistance tomatch the impedance of the probe and cables and minimized resistance tominimize the voltage drop and the power dissipated by the resistor.

What is desired, therefore, is a probing system and method havingminimal resistance to minimize voltage drop and power dissipationcombined with adequate resistance to match the impedance of the probeand cables to minimize modulation signal distortion when a directcurrent and a modulated signal are simultaneously applied to a DUT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a first embodiment of a combinedsignal probing system.

FIG. 2 is a schematic illustration of a second embodiment of a combinedsignal probing system.

FIG. 3 is a perspective view of a first embodiment of a combined signalprobe.

FIG. 4 is a section view of the combined signal probe of FIG. 3 takenalong line A—A just after contact has been made between the probe tipand a contact pad of a device-under-test.

FIG. 5 is a fragmentary view corresponding to FIG. 4 showing how theprobe tip moves in relation to the body of the probe in response to adownward shift of the body in relation to the DUT.

FIG. 6 is a section view of the combined signal probe of FIG. 3 takenalong line B—B.

FIG. 7 is an enlarged perspective view of the probe tip of the combinedsignal probe of FIG. 3.

FIG. 8 is a bottom view of the probe tip of FIG. 7.

FIG. 9 is a schematic illustration of a third embodiment of a combinedsignal probing system.

FIG. 10 is a perspective view of a second embodiment of a combinedsignal probe having separated direct current and modulation signalcontact tips.

FIG. 11 is a bottom view of the probe tip of the probe of FIG. 10 havingseparated contact tips.

DETAILED DESCRIPTION OF THE INVENTION

On-wafer testing of certain devices requires the simultaneousapplication of direct current and a modulated signal to conductivecontact pads on the wafer that correspond to the device-under-test(DUT). For example, to test a laser diode on a wafer, a DC current and amodulation signal are simultaneously applied to a contact pad for thediode. The DC current provides the power for generating light and themodulation signal varies lasing intensity to produce a modulated lightoutput. For testing, the modulation signal is typically a sweptfrequency sinusoid (AC) or a wide bandwidth pulsed waveform. Whentesting a laser diode, the dynamic resistance of the operating diodewhich is typically on the order of five ohms is the impedance seen bythe modulation signal. On the other hand, the typical microwave probeand cable has an impedance of approximately 50 ohms. As a result of thesignificant impedance mismatch, the modulation signal will be distorted.Some test instrumentation, such as a Vector Network Analyzer (VNA), cancompensate for some signal distortion, but distortion compensation isnot possible with other instrumentation. Further, the distortion maycause attenuation of the signal at certain frequencies adverselyaffecting the dynamic range and accuracy of measurements even whenperformed with a VNA.

To improve the impedance matching and reduce distortion of themodulation signal, a resistor can be incorporated into the tip of theprobe. For example, an impedance matching resistor with a value of 45ohms in series with the five ohm dynamic resistance of a typicaloperating laser diode provides a satisfactory impedance match withtypical microwave probes and cables and substantially reduces signaldistortion. However, since the modulation signal and the DC current aresuperimposed on the same conductor, both signals must pass through theseries impedance-matching resistor resulting in a voltage drop anddissipation of power substantially equal to the product of theresistance and the square of the DC current. For devices requiring highcurrent levels, the power that is dissipated by the resistor issubstantial. However, to minimize parasitic series inductance and shuntcapacitance of the probe, the size of the resistor must be minimized,limiting its capacity to dissipate power. Probe performance must becompromised to satisfy the competing requirements imposed on theimpedance-matching resistor. Decreasing the resistance reduces theamount of power that is dissipated by the resistor but increases theimpedance mismatch and the signal distortion. On the other hand,increasing the size of the resistor increases its capacity to dissipatepower but also increases its inductance and capacitance and,consequently, the distortion of the modulation signal. The presentinventor concluded that the performance of a probe applying a combinedDC current and modulation signal to a DUT could be improved if thesignal paths could be separated so that the modulated signal wastransmitted over a signal path having a matched impedance while the DCcurrent flow was transmitted over a signal path with minimal resistanceto minimize loss and power dissipation.

Referring in detail to the drawings wherein similar parts of theinvention are identified by like reference numerals, and moreparticularly to FIG. 1, an on-wafer probing system 20 for testing a DUT22 (for example, a laser diode) requiring simultaneous application of DCcurrent and a modulated signal includes a DC power supply 24 and a highspeed test instrument 26 such as a VNA or a Bit-Error-Rate testersupplying a modulation signal. The DC power supply 24 is connected tothe DC port 28 of a bias-tee 30. A bias tee 30 is a device used tosuperimpose a direct current and a modulation signal or for extracting adirect current component from a combined signal without affecting thehigh frequency component of the signal. The combined DC/RF port 32 ofthe bias tee 30 is connected to a first input port 42 of the combinedsignal probe 40. The modulated signal or radio frequency (RF) port 34 ofthe bias tee 30 is connected to a termination resistor 36 to provide animpedance matched termination for the modulation signal imposed on theconductor connecting the bias tee and the combined signal probe. Thefirst input port 42 of the combined signal probe 40 is conductivelyconnected to a signal contact tip 44 that is arranged to selectivelyengage a signal contact pad 46 of the DUT 22 (a laser diode).

On the other hand, the modulation signal, generated by theinstrumentation 26 of the probing system 20, is transmitted to a DCblock 50. The DC block 50 comprises, generally, series capacitance thatblocks the flow of DC current into the instrumentation 26 over theconductor that connects the instrumentation 26 to the combined signalprobe 40. From the DC block 50 the modulation signal is transmitted tothe second input port 48 of the combined signal probe 40 which isconductively connected to a first port 53 of an impedance-matchingresistor 52. The second port 54 of the impedance matching resistor 52 isconductively connected to the signal contact tip 44 of the probe. Asecond contact pad 56 of the DUT 22 is grounded 58 through a groundcontact tip 45 of the combined signal probe 40. The signal contact tip44 and the ground contact tip 45 are arranged to simultaneouslyselectively engage, respectively, the signal contact pad 46 and theground contact pad 56 of the DUT 22.

In the probing system 20, the modulation signal sees a terminationcomprising the impedance-matching resistor 52 in series with theparallel combination of the dynamic resistance of the DUT and theimpedance seen looking from the first input port 42 back toward the biastee 30. If the termination resistor 34 has a resistance matching theimpedance of the connection between the bias tee 30 and the combinedsignal probe 40 then the impedance at the DUT is equal to the impedanceof the connection between the bias tee and the combined signal probe.Typically, the connection between the bias tee 30 and the combinedsignal probe 40 comprises coaxial cable with an impedance ofapproximately 50 ohms. Since the impedance of the DUT is typicallysubstantially less (typically, five ohms for a laser diode) than theimpedance looking into the coaxial cable connection toward the bias tee30, the parallel combination of impedances is dominated by the smallerimpedance and the modulation signal path is approximately matchterminated, minimizing distortion of the modulation signal. On the otherhand, the signal path of the DC current powering the DUT does not passthrough the impedance-matching resistor 52 so losses in theimpedance-matching resistor are minimized. The probing system 20 permitsa modulation signal to be transmitted to the DUT 22 over a first signalpath that includes an impedance matching resistor 52 while a directcurrent is simultaneously transmitted to the DUT over a second signalpath that does not traverse the impedance matching resistor.

Referring to FIG. 2, in a second embodiment of the probing system 60,the instrumentation 62 is the source of an offset modulation signal. Thecombined DC current and modulation signal are transmitted to a combinedsignal port of a first bias tee 64. In the first bias tee 64, the DCcurrent and modulation signal components are separated. The modulationsignal is transmitted to the first input port 48 of the combined signalprobe 68 which is conductively connected to the first port of animpedance matching resistor 70. The second port of the impedancematching resistor 70 of the probe 68 is connected to a modulation signalcontact tip 72 arranged to selectively engage the signal contact pad 46of the DUT 22.

The DC current is transmitted from the first bias tee 64 to a secondbias tee 66. The combined signal (DC/RF) port of the second bias tee 66is connected to the second input port of the probe 42 which isconductively connected to a DC signal probe 76 arranged to engage thesignal contact pad 46 of the DUT 22 when the modulation signal probe 72is in engagement with the signal contact pad. An impedance matchedtermination for the modulation signal imposed on the conductorconnecting the second bias tee 66 to the DC signal probe 76 is providedby a termination resistor 74 connected to the RF port of the second biastee 66. As in the first embodiment, the modulation signal is applied tothe signal contact pad 46 of the DUT 22 over a distortion minimizingimpedance matched signal path while the DC current is simultaneouslyapplied to the signal contact pad over a signal path that does notinclude the impedance matching resistor and, therefore, minimizes powerdissipation.

Referring to FIG. 9, in still another embodiment of the probing system80, the DC current is generated by a power supply 24 and transmitted tothe DC signal contact tip 76 of the combined signal probe 68 over asignal path including inductance represented by the inductor 82. Themodulation signal, generated by the instrumentation 26, is transmittedthrough the DC block 50 to the first input port 48 of the combinedsignal probe 68 which is conductively connected to the first port of theimpedance matching resistor 70. The second port of the impedancematching resistor 70 is conductively connected to the modulation signalcontact tip 72 which is arranged to engage the signal contact pad 46 ofthe DUT 22 when the DC signal contact tip 76 is in contact with thecontact pad. The flow of DC current toward the instrumentation 26 isblocked by the capacitance of the DC block 50. At the frequency of themodulation signal, the impedance presented by the inductance 82 issubstantially greater than impedance of the DUT, substantially blockingthe passage of the modulation signal toward the power supply whilepermitting the DC current to flow, substantially unimpeded, to thecombined signal probe 68 over a signal path that bypasses the impedancematching resistor 70. Distortion of the modulation signal is minimizedby the impedance matching in the modulation signal path while power lossis minimized by avoiding the flow of current through the impedancematching resistor 70.

Referring to FIGS. 3, 4, 5, and 6, an exemplary first embodiment of acombined signal wafer probe 100 constructed in accordance with thepresent invention is designed to be mounted on a probe-supporting member102 of a wafer probe station so as to be in suitable position forprobing a DUT, such as an individual laser diode component on a wafer104. In this type of application, the DUT is typically supported undervacuum pressure on the upper surface of a chuck 106 that is part of theprobing station. Ordinarily an X-Y-Z positioning mechanism is provided,such as a micrometer knob assembly, to effect movement between thesupporting member and the chuck so that the tip assembly 110 of theprobe can be brought into pressing engagement with contact pads 108 onthe DUT that correspond to the particular component requiringmeasurement.

With respect to its overall construction, the wafer probe 100 includes aprimary support block 112 which, in the illustrated embodiment, is madeof brass and which is suitably constructed for connection to theprobe-supporting member 102. To effect this connection, a round opening114 that is formed on the block is snugly fitted slidably onto analignment pin (not shown) that upwardly projects from theprobe-supporting member, and a pair of fastening screws 116 are insertedinto a corresponding pair of countersunk openings 118 provided on theblock for screwing engagement with the probe-supporting member, eachwithin a respective threaded opening formed on that member.

As illustrated in FIG. 1, the first embodiment of the exemplary combinedsignal wafer probe 100 includes a first input port 120 and a secondinput port 122 which, in the preferred embodiment depicted, comprisespark-plug type, K-connectors. This connector enables the externalconnection of an ordinary coaxial cable to the input ports 120, 122 ofthe wafer probe. The connection of a coaxial cable to the first inputport 120 permits a well-shielded high frequency transmission channel tobe established between the probe and an attached measuring instrument26. Likewise, a shielded high frequency transmission channel between thebias tee 66 and the combined signal wafer probe 100 is established byconnecting a coaxial cable between the second input port 122 of theprobe and the combined (DC/RF) port of the bias tee. If desired, othertypes of connectors can be used such as a 2.4 mm connector, a 1.85 mmconnector or a 1 mm connector. The combined signal wafer probe 100provides low-loss transmission paths having a controlled impedancecharacteristic from the input ports 120, 122 down to the contact tipassembly 110. The tip assembly 110 of the wafer probe is of particularlyrugged construction and able to withstand in excess of 500,000 separatecontact cycles without maintenance or repair. At the same time, the tipassembly is able to readily adapt to non-planar contact pad surfaces ofa DUT on a wafer 104.

In the preferred embodiment shown in FIG. 4, a semirigid coaxial cable124 is electrically connected at its rearward end to the K-connector ofthe first input port 120. Referring also to FIG. 7, this coaxial cable124 includes an inner conductor 126, an inner dielectric 128 and anouter conductor 130 and is preferably of phase-stable, low-loss type.Similarly, as illustrated in FIG. 6, a semirigid coaxial cable 150 isconnected at its rearward end to the K-connector at the second inputport 122.

To prepare the rearward ends of the cables 124, 150 for connection tothe appropriate K-connector, the rearward end is stripped to expose theinner conductor, and this inner conductor is temporarily held inside adummy connector while the adjacent outer conductor is soldered within abore 140, 152 formed in the primary support block 112. A recess 142 thatis formed in the block below this bore provides access to facilitate thesoldering process. The dummy connector is then removed and theK-connectors are screwably installed in threaded openings 144 formed onthe block above the bore so as to effect electrical connection betweenthe connectors and the coaxial cables 124, 150. A thread lockingcompound may be applied to the threads of the K-connectors prior totheir installation to ensure a tight physical connection.

Referring to FIGS. 4 and 5 together, the forward end 146 of the cable124 remains freely suspended and, in this condition, serves as a movablesupport for the probing end 110 of the probe. Before being connected tothe K-connector of the first input port 120, the cable 124 is bent alongfirst and second intermediate portions in the manner shown in FIG. 4 sothat an upwardly curving 90° bend and a downwardly curving 23° bend,respectively, are formed in the cable. A tube 154 of semi-flexiblemicrowave-absorbing material is slidably inserted over the protrudingend of the coaxial cable 124. One material used for forming the tube iscomprises iron and urethane. The bottom of the rigid support block 112is covered with a soft and flexible sheet 156 formed ofmicrowave-absorbing material so as to provide a cushioning layer alongthe bottom of that block. An example of material of suitable type forthis purpose is a filled silicon rubber containing iron. Themicrowave-absorbing components on the exemplary probe, that is, therigid support block 112, the flexible sheet 156 and the semi-flexibletube 154, cooperatively serve to substantially reduce the levels ofmicrowave energy that travel along the outer conductor 130 of thesemirigid cable 124 and other exterior probe structures.

As illustrated in FIG. 4, the combined signal probe 100 is positioned sothat the probe tip 110 is brought into contact with the contact pad 108of the DUT. After probe tip 110 is brought into pressing engagement withits corresponding contact pad the vertical spacing between the probe 100and the device-under-test is then reduced even further, as depicted inFIG. 5, causing the coaxial cable 124 to bend and causing the contacttip to wipe across the surface of the corresponding contact pad 108, asindicated.

Prior to its connection to the K-connector of the input port 120, asemicylindrical recess 202 is formed in each of the cables 124, 150adjacent their forward ends as shown in FIG. 7. This recess is formed bymaking a longitudinal cut through the cable and by making a transversecut at the end of the longitudinal cut. In accordance with thisprocedure, a semicylindrical portion of the outer conductor 130, theinner dielectric 128, and the inner conductor 126 are removed, as sothat the remaining portions of these elements together form a flat shelf204 that extends to the forward end of the cable as well as a back face206 that extends crosswise in relation to the length of the cable.

Referring to FIGS. 7 and 8, at the probing end of the exemplary probe,an inner conductive finger 250 is connected to the inner conductor 126of the cable 124 and a pair of outer conductive fingers 252 a, 252 b areconductively connected to the adjacent outer conductor 130 so as to forma signal-ground conductor configuration. While the exemplary probe tipincludes a pair of outer conductive fingers, the probe tip can beconstructed with a single outer finger. Referring also to FIG. 8, whichshows a bottom view of the probing end 146, each respective fingerincludes a cantilevered portion 254 that extends past the forward end256 of the cable 124. The cantilevered portions 254 are arranged intransversely spaced apart relationship to each other so as tocooperatively form a controlled impedance transmission line in orderthat a low-loss transition can be made between the respective conductors130 and 126 of the cable 124 and the respective pads on thedevice-under-test.

To a certain extent, the spacing between the respective fingers 252 a,252 b, and 250 is determined by the geometry of the device contact padsand the cable. For example, in relation to the distal ends of therespective fingers, the pitch or centerline-to-centerline spacing 270between adjacent pairs of the fingers is selected in order to match thepitch 270 of the contact pads on the device-under-test. The distal endsof the pair of fingers may be set apart at a pitch of 6 mils in order tomatch the 6 mil pitch of 2 mil square contact pads on adevice-under-test. (It is also customary for the pad-to-pad pitch to beset at other values such as 4, 5, 8 or 10 mils). On the other hand,proximate the back face 204 of the cable 124, the pitch between adjacentfingers is selected to correspond with the pitch between the exposedface of the inner conductor 126 and the adjacent exposed face of theouter conductor 130 of the cable 124.

Aside from the dimensions just mentioned, the principal criteria used inselecting the respective dimensions and relative spacing of the fingers250, 252 a,252 b is the desired establishment of a low-loss transmissionline between the respective conductors 126 and 130 of the cable and therespective pads on the DUT.

The distal end 272 of the central finger 250 comprises the signalcontact tip 44 and the distal ends of the outer fingers 252 a, 252 b areconnected to ground through the outer conductor 130 of the coaxial cable124. The three fingers are attached to the cable near the end of thecable 124 by a non-conductive adhesive 276, such as an epoxy adhesive.At the end of the fingers remote from the contact tips, the centerfinger 250 is affixed to the center conductor 126 and the outer contacttips 252 a,252 b are affixed to the outer conductor 130 of the coaxialcable. Referring to FIG. 8, to incorporate an impedance matchingresistor 52 in series between the center conductor 126 of the coaxialcable 124 and the signal contact tip 44, an aperture 280 is drilled inthe body of the central finger 252 at a position between the solderedconnection 278 and the non-conductive attachment 276 of the centerfinger to the cable. The aperture 280 is of such size and depth as tosever the center finger 250 and the center conductor 126 of the cable124. A resistor 282 deposited on a ceramic substrate is inserted intothe aperture and bonded in the aperture 280. A conductive adhesive 284connects the forward portion of the center finger 250 to the rearwardportion which is soldered to the center conductor 126.

As illustrated in FIG. 6, the coaxial cable 150 connected to the secondinput port 122 of the combined signal probe 100 follows a path throughthe probe substantially paralleling that of the coaxial cable 124connected to the first input port 120. The coaxial cable 150 from thesecond input port 122 terminates adjacent to the probe end of thecoaxial cable 124. A jumper 300 is affixed to the center conductor 302of the coaxial cable 150 and to the center finger 250 projecting fromthe coaxial cable 124. As a result, DC current from the power supply 24transmitted through the second input port 124 of the combined signalprobe 100 is conducted directly to the signal contact tip 44 at the end272 the center finger 250 over a signal path that does not pass throughthe impedance matching resistor 52.

Referring to FIG. 10, a second embodiment of the combined signal probe500 comprises, generally, the primary support block 112 for mounting theprobe and the K-connectors of the input ports 120, 122 and supportingcoaxial cables 502, 504 connecting the input ports and the probe's tipassembly 506. The tip assembly 506 comprises individual contact tipassemblies for each the direct current 508 and the modulation signal 510signal paths. Referring to FIG. 11, the contact tip assembly for themodulation signal 510 comprises a modulation signal tip 512 and at leastone ground contact tip 514 arranged to simultaneously engage the DUT'ssignal and ground contact pads, respectively. The impedance matchingresistor 516 connects the modulation signal contact tip 512 to themodulation signal path at the center conductor of the coaxial cable 504.The direct current tip assembly 508 comprises a contact tip connected tothe center conductor 520 of the coaxial cable 502 which provides thesignal path for the direct current. The direct current contact tip 508is arranged to contact the signal contact pad of the DUT when themodulation signal contact tip 510 and the ground signal contact tips 514are brought into engagement with the signal and ground contact pads ofthe DUT. The direct current contact tip 508 may have a needle-likestructure to provide an inductive element 82 that passes the directcurrent with minimal resistance but exhibits a high impedance to signalsat the frequency of the modulation signal.

The combined signal probe, probe system, and the method of probingpermits a DC current and a modulation signal to be transmitted inparallel for combination at the probe signal contact tip so thatimpedance matching can be applied to the modulation signal path toreduce signal distortion while resistance is minimized in the path ofthe DC signal to minimize voltage drop and power dissipation.

The detailed description, above, sets forth numerous specific details toprovide a thorough understanding of the present invention. However,those skilled in the art will appreciate that the present invention maybe practiced without these specific details. In other instances, wellknown methods, procedures, components, and circuitry have not beendescribed in detail to avoid obscuring the present invention.

All the references cited herein are incorporated by reference.

The terms and expressions that have been employed in the foregoingspecification are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims that follow.

1. A probing system for interconnecting a source of a modulated signaland a source of a direct current to a terminal of a device under testhaving a device impedance, said probing system comprising: (a) acapacitor; (b) a resistor connected in series with said capacitor, saidresistor and said capacitor interconnecting said source of saidmodulated signal and said terminal of said device under test; and (c) aninductor interconnecting said source of direct current and said terminalof said device under test.
 2. The probing system of claim 1 wherein animpedance of said inductor is substantially greater than said deviceimpedance at a frequency of said modulated signal.
 3. The probing systemof claim 1 wherein an impedance of said resistor is substantially equalto an impedance of an interconnection of said resistor and said sourceof said modulated signal.